ABSTRACT: The ability of monovalent IgG and F(ab')2 antivenoms to neutralize lethality, phospholipase A2, and coagulant activities induced by Daboia siamensis venom was studied. Both antivenoms were produced from the same batch of hyperimmune horse plasma and were adjusted to the same potency against the lethal effect of D. siamensis venom in experiments involving preincubation of venom and antivenom. Intact neutralization experiments involving independent injection of venom and antivenoms showed that the F(ab')2 antivenom was slightly more effective. Significant differences in favour of F(ab')2 antivenom were observed with respect to neutralization of phospholipase A2 and coagulant activities. Both IgG and F(ab')2 antivenoms were able to activate human complement in vitro. IgG antivenom had a significantly higher anticomplementary activity than F(ab')2 antivenom.